Wood plastic composites (“WPC”) refer to any composite that contains wood such as wood flour or wood fiber and plastic such as polyethylene, polypropylene, polyvinyl or polyvinyl chloride. The WPC or “synthetic lumber” industry has grown dramatically in the past ten years in North America. The main applications include decking, railing, boardwalk, porch, park bench seats and wood trim which have accounted for more than about five million U.S. dollars in sales in 2003. The use of wood plastic composites in place of traditional wood materials is driven by the characteristics of better resistance to moisture and rot, better resistance to insects, less routine maintenance, no cracking, splitting, warping or splintering.
Synthetic lumber has been used as a substitute for wood in areas where wood can deteriorate quickly due to environmental conditions. Although in the past, the commercialization of synthetic lumber was limited by costs, modern recycling techniques and low cost extrusion manufacturing capability have permitted greater penetration by polymer-fiber composite materials into the commercial and residential markets. One such product manufactured under the trademark TREX, by Trex Company, LLC, Winchester, Va., consists of a polyethylene-wood fiber blend which is extruded into board dimensions for decking applications. Polyethylene-wood composite boards in 5/4 inch thicknesses have sufficient rigidity to be used as decking planks, but typically are not recommended for structural wood substitutes, such as the lattice supporting structure often used under decks.
Polyethylene composites are attractive because they permit screw fasteners to “countersink”, such that the heads of the screws bury or at least become flush with the board surface, without predrilling. Synthetic wood products, like TREX decking, are weather resistant and relatively maintenance free. Once installed, they resist splintering and warping normally associated with wood boards. They are also characterized by “color weatherability”; for example, the TREX product initially is a light coffee brown color and converts to a weathered gray appearance when exposed to rain water and sunlight. Accordingly, the TREX decking color fades and is not permanent.
In addition to polyethylene, other plastics have been suggested for use in the manufacture of synthetic wood products. Polyvinyl-chloride (“PVC”) thermoplastics have been used in combination with wood fibers to make extruded materials for use in decking, windows and doors. See U.S. Pat. No. 5,486,553 assigned to Andersen Corporation, and Stucky et al., U.S. Pat. No. 6,344,268, assigned to CertainTeed Corp., and incorporated herein by reference.
Color is a key component in the appearance of wood plastic composites. Most successful companies have a product line which includes four colors, namely, red, dark brown, tan and gray, to duplicate the main premium woods, for example, mahogany, red cedar, oak, etc. Companies with only one or two colors enjoy only limited market share. The ideal color mix is estimated to be 70 percent dark color such as dark brown or red and 30 percent light or translucent color such as gray or tan. The use of industrial pigments to obtain dark colors represents a significant component of the raw material cost. Industrial pigments made of iron oxide usually cost about $1.50 per pound while other pigments can be as expensive as $4.00 per pound, which is a multiple of the cost of the resin.
Wood plastic composites with a low wood flour content tend to have a very plastic appearance, while wood plastic composites with a high wood flour content usually have a better touch and appearance due to the wood particles, which appear at the surface of these products.
The main advantages of organic wood flour is its availability, light weight and low cost. Wood flour is also less abrasive to processing equipment than most conventional fillers. For many years the plastic industry was reluctant to use wood or other natural cellulosic fillers, due to their low bulk density, low thermal stability and tendency to absorb moisture. While this perspective has changed somewhat in the last ten years, due to the success of several wood plastic composite products, wood flour and wood fiber are still sensitive to moisture absorption, fungi attack and decay. High wood loadings of generally between 30 and 70 wt. % result in a surface which is covered by many unprotected wood particles, which are not encapsulated by plastic, and thus are subject to attack by decay, fungi and moisture.
Complete encapsulation of wood flour by plastic to prevent moisture absorption and fungal attack is not practical for cost reasons, generally, it would require a high percentage of plastic to fully encapsulate the wood particles, and aesthetic reasons, the resulting finish would look too much like a plastic. Wood polymer composites with too much plastic feel more like plastic than wood and are not appreciated by customers.
Successful companies have developed wood polymer composites with high wood loading of generally between 50 and 60 wt. %. To account for wood sensitivity to moisture absorption and bacterial growth, manufacturers of high wood loading building components rely on the use of extensive amounts of anti-bacterial agents to limit the growth of fungi and algae at the surface of these products. The use of these anti-bacterial agents does not guarantee that the products will be maintenance free and does not prevent infiltration of water into the product, nor prevent physical and photo-chemical degradation. Product appearance is likely to change within a few months or years, and colors may be affected first.
The use of mineral fillers in plastic composite lumbers is not new, Century Board, Inc., a licensee of Ecomat, Inc. has developed a plastic composite lumber that contains 70 wt. % fly ash. The resin is a modified polyester-polyurethane thermoset that can be foamed to produce products with similar density, stiffness and toughness of wood products. See U.S. Pat. Nos. 5,604,266; 5,508,315; and 5,369,147, which are hereby incorporated by reference. The Ecomat building materials describe the use of fly ash and several other mineral fillers with a polyester-polyurethane resin to produce foamed plastic composites for building applications. However, fly ash derived from waste incinerators, which is some of the most inexpensive fly ash available, is not generally safe and has a high content of heavy metals.
Accordingly, there remains a need for a building material which more closely simulates wood products, or which has heretofore previously unavailable aesthetic properties.